Diathermy having meter circuit indicating true power drawn by a patient

ABSTRACT

A diathermy having a vacuum tube tuned plate-tuned grid oscillator including a coil for delivering high-frequency signals to a patient undergoing treatment. A meter coupled between the plate circuit for the tube and a source of potential therefor. A bypass circuit is coupled across the meter for bypassing a preselected amount of current causing an improved direct indication of power drawn by a patient undergoing treatment.

United States Patent 2,103,440 12/1937 Weissenberg Jerome W. Davls Pasadena, Calll.

July 30, 1969 Nov. 16, 1971 Mettler Electronics Corporation Inventor Appl. No. Filed Patented Assignee DIATHERMY HAVING METER CIRCUIT INDICATING TRUE POWER DRAWN BY A PATIENT 10 Claims, 1 Drawing Fig.

US. Cl 128/422, 324/126 Int. Cl A6ln 1/08 Field of Search 128/404, 405, 413, 421, 422, 303.13, 303.14, 303.18, 303.19; 323/38; 324/126 References Cited UNITED STATES PATENTS Senanke Mittlemann 2/1952 Gufoffet al 1/1954 Touzel FOREIGN PATENTS 5/1963 Great Britain Primary Examiner-William E. Kamm Attorney-Christie, Parker & Hale ABSTRACT: A diathermy having a vacuum tube tuned platetuned grid oscillator including a coil for delivering highfrequency signals to a patient undergoing treatment. A meter coupled between the plate circuit for the tube and a source of potential therefor. A bypass circuit is coupled across the meter for bypassing a preselected amount of current causing dergoing treatment.

. an improved direct indication of power drawn by a patient un- PATENTEnunv 1s WI 3, 620.221

29k I l l .24 5 9 94 My JEROME W A TTOIQN Y5 DIATHERMY HAVING METER CIRCUIT INDICATING TRUE POWER DRAWN BY A PATIENT CROSS REFERENCES TO RELATED APPLICATIONS Description is given herein of a diathermy oscillator circuit which is disclosed and claimed in a U.S. Pat. application entitled SHORT WAVE DlATl-IERMY CIRCUIT given Ser. No. 846,012 tiled on the same date as this application in the name of Hal C. Mettler.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to diathermies and, more particularly, to a circuit for zeroing the power meter in a diathermy.

2. Description of the Prior Art Diatherrnies are known which generate high frequency signals for treating a patient undergoing treatment. Most known diathermies have a vacuum tube oscillator. Metering circuits are used to indicate the power drawn by the plate circuit of the oscillator. In general, the plate current-metering circuit of prior art diathermies do not provide a direct indication of the amount of power being supplied to the patient because of the losses involved in the methods of applying and controlling the power delivered to the patient.

SUMMARY OF THE INVENTION The present invention overcomes the aforementioned disadvantages by providing a special bypass circuit around the meter. An oscillator delivers signals to a patient coil. The bypass circuit has an input coupled to the oscillator input and restricts the current flow through the meter to that which represents the increased power delivered to a patient undergoing treatment responsive to the input signal to the oscillator.

A preferred embodiment of the present invention is in a diathermy which employs a vacuum tube having a screen grid which is used to control the output power of the diathermy by variation of the screen supply voltage. Means is provided for restricting the current flowing through the meter to that which represents the high frequency power being delivered to the patient undergoing treatment. In general, the high frequency power delivered to the patient is directly related to the rise in plate circuit current above its idling value under the condition of no load. The metering meter-zeroing circuit restricts the current flow through the meter to that which represents the true power delivered to a patient undergoing treatment. The preferred embodiment of the invention overcomes two specific difficulties. First, it bypasses the portion of idle current around the meter which is present when there is no patient load. Second, the idle current changes as screen-control voltage changes and the preferred embodiment of the invention compensates for this change in idle current as well.

In another preferred embodiment a scale-expanding circuit is provided for the meter circuit, in combination with the meter-zeroing circuit, for a desired range of registration of the meter. The scale-expanding circuit includes a bypass circuit around the meter. A nonlinear conductive device is coupled to the bypass circuit and the meter through which current to the oscillator may pass. The nonlinear conductive device changes internal impedance as current therethrough changes causing a different amount of current to flow through the bypass circuit and the meter circuit as the power delivered to the patient increases.

BRIEF DESCRIPTION OF THE DRAWINGS The drawing is a schematic diagram of a diathermy circuit embodying the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Consider first the oscillator shown in the drawing. The circuit includes a helical induction coil 30 for applying the shortwave diathermy signals directly to the patient undergoing treatment.

The center tap of the coil 30 is coupled to the B+ potential output of a power supply 50. The ends of the coil 30 are connnected across two series connected capacitors 32 and 34. The coil 30 and capacitors 32 and 34 form a tuned tank circuit for the plate circuit of the oscillator. The junction between the two capacitors 32 and 34 is connected to ground potential. It will be evident to those skilled in the art that each capacitor 32 and 34 may comprise two or more capacitors connected in parallel to make up the desired capacitance.

The two sides of the tank circuit 29 are labeled 29(0) and 29(b) and are connected to the plate electrodes of a pair of beam vacuum tubes 36 and 38, respectively. Each tube has a plate, a screen grid, a control grid, and a cathode. It should be understood that the invention is not limited to use of a beam tube but any tube having at least the aforelisted electrode and similar characteristics may be used.

The cathodes of the pentodes 36 and 38 are connected together to the B- potential output of the power supply 50.

The tuned grid circuit is shown generally at 41 in FIG. I. The tuned grid circuit includes an inductive coil 39-40 coupled between the control grids of the two tubes. This coil is center-tapped and the center tap is connected to the cathodes through a resistor 42 which is provided to limit the current passing through the grid coil 39-40. The resistor 42 is a very high impedance and hence the cathodes are not directly connected to the center tap. The grid to cathode capacitances (not shown) of the tubes 36,38 are in series across'the ends of the grid coil 39-40 to form the tuned grid circuit.

Resistors 44 and 46 connect the screen grids of the pentodes 36 and 38, respectively, to a source of screen-control voltage. The screen control is derived from a potentiometer 102 as will be explained more completely hereinafter.

The upper and lower halves of the oscillator circuit shown in the drawing each form a conventional tuned plate, tuned grid circuit of the type described in the book entitled Radio Engineering" by Terrnan published by the McGraw-Hill Book Company, Inc., New York and London, 1932. The two halves are connected together in a conventional push-pull fashion well known in the oscillator circuit art. Accordingly, a description of the operation of the oscillator circuit will not be given, and a more complete understanding thereof can be had with reference to the abovereferenced book entitled Radio En"- gineering".

However, the control for the power delivered to the patient through the helical coil 30 should be noted. The power is controlled by the voltage output from the source of screen control voltage which includes the potentiometer 102. An alternate form of power control is by varying the spacing between the patient and the output coil, and this method is sometimes used with the present unit.

In one embodiment of the invention the screen control voltage delivered by the potentiometer 102 varies between 0 and 140 volts, causing power to be delivered by the coil 30 to a patient varying between 0 and watts.

The screen resistors 44 and 46 are of considerable importance. It should be noted that the screen resistors 44 and 46 are each of such a value that the screen grids retain their ability to control the power to the circuit and yet large enough to allow the fundamental shortwave frequency to be fed back from the plate to the control grid and thereby sustain oscillation in the circuit. Most known oscillator circuits of this type are only triodes and do not utilize a screen grid for control. By utilizing a tube with a screen grid and connecting the screen resistors as shown, significantly improved results are obtained by eliminating the undesirable harmonics which prevented diathermy circuits of this type from being useful in the past and allowing precise control over the power output.

The values of the resistors 44 and 46 do not appear to be critical and it has been found that resistors may be selected by placing a variable resistor in place of the resistors 44 and 46 and varying them until the desired power output is obtained. However, it has been found that the undesirable harmonics are filtered out better as the value of the resistors 44 and 46 is increased up to the point where the screens lose control of the power output.

With the oscillator circuit in mind consider the meter circuit 53 and the meter-zeroing circuit 70. The B+ output of the power supply 50 is connected to the coil 30 through a meter circuit 53 and a meter-zeroing circuit 70. The symbol B+ is used to label the output of the power supply 50 and the input to the oscillator at the coil 30 because the voltage drop between the two points is negligible.

The circuit 53 is actually connected serially in the line between the B+ output of the power supply 50 and the pancake coil 30 in the oscillator. Thusit will be evident that the plate current passes through the circuit 53. A resistor 54 is connected in series with the meter 52. A resistor 56 is connected in series with a diode 58. The series-connected elements 56 and 58 are connected in parallel with the meter 52 and resistor 54. The diode 58 is poled with its anode electrode connected to the resistor 56.

In operation, the coil 30 is brought near a patient causing the current in the plate circuit of the oscillator to increase. Thus, current through the circuit 53 increases. The increase in current is directly proportional to the amount of power being delivered to the patient. The diode 58 has a forward potential below which it is essentially nonconductive. The forward potential of the diode 58 and the value of the resistor 54 are selected to so that the voltage drop across the meter and resistor 54 will bias the diode 58 into the conductive condition when the power indicated on the meter arrives at a preselected value (Le, 20 watts). As the power, and hence the plate current, increases above that conductive condition of diode 58, the current divides between the resistor 56 path and the meter and resistor 54 path and hence reduces the amount of current passing through the meter 52. In this manner, the upper end of the scale on the meter 52 is compressed.

The present invention principally centers around the zeroing circuit 70. The purpose of the zeroing circuit 70 is to cause the meter 52 to indicate power when there is no patient in the field of the coil 30 and to allow a direct indication of power being delivered to a patient as the patient is brought into the field of the coil 30. The need for the zeroing circuit can be explained as follows: When a patient is out of the field of the coil 30 and hence is not drawing power, the current in the plate circuit of the oscillator, and hence through the meter circuit 53, is referred to as the idle current. Additionally, as the screencontrol voltage is changed the idle current changes. As a result it is necessary to correct the current flowing through the meter circuit 53 for the changing idle current so that the current flowing through the meter circuit 53 is truly proportional to the increase in current flowing in the plate circuit due to power drawn by a patient. In this manner, the meter 52 can be made to register the true power drawn by a patient. The zeroing circuit 70, therefrom, in effect provides a sliding zero.

Consider the details of the zeroing circuit 70. A bypass circuit is provided around the meter 53. The bypass circuit includes the elements 72 through 85. A PNP-type transistor 72 has its collector connected to the coil 30 of the oscillator and its emitter coupled through a resistor 74 to the B+ output of the power supply 50. The diodes 80 and 82 are provided for biasing the collector of the transistor 72 to the correct potential. The series-connected elements 78 and 76 are a variable resistor and a diode which are used to adjust the gain of the transistor 72 to the correct level. The elements 84 and 85 are resistors which are connected between the base of the transistor 72 and a +V potential output of the power supply 50 for biasing the base of the transistor 72. The resistors 84 and 85 are selected and the gain of the transistor 72 is adjusted by means of the variable resistor 78 so that when the wiper arm of the potentiometer 102 is positioned at the B- output of the power supply 50, the meter 53 registers zero watts.

The wiper on the potentiometer 102 controls the voltage applied to the screen grid of the tubes 36 and 38 and hence the power delivered by the coil 30. A resistor 100 is connected in series with the potentiometer 102 between the +V and the B- outputs of the power supply 50.

A monitoring circuit is provided for controlling the bias on the base of the transistor 72 as the screen voltage for the oscillator is increased. In this manner the bypass circuit elements 72 through cause the right amount of current to be bypassed around the meter 52 so that the current passing through the meter 52 accurately reflects the current drawn by a patient from the pancake coil 30. The monitoring circuit includes the elements 86 through 98. The transistor 86 is a NPN-type transistor having a base electrode connected to the wiper of the potentiometer 102 and its collector electrode connected through a resistor 98 and a Zener diode 87 to the junction between the resistors 84 and 85 (in the bypass circuit). The emitter electrode of the transistor 86 is connected through a resistor 96 to the junction of the resistors 44 and 46 in the oscillator. With the wiper of potentiometer 102 adjusted at the B-- output of the power supply 50 the screen grids of the oscillator tubes 36 and 38 are biased to the point where minimum power will be delivered by the helical coil. Under these conditions the circuit is such that a very small and negligible amount of current flows through the Zener diode 87, the resistor 98 and the collector to emitter circuit of the transistor 86. The purpose of the Zener diode 87 is to provide a high-voltage drop between the junction of resistors 84 and 85 and the collector of the transistor 86 to prevent a reverse breakdown between the base and collector electrodes of the transistor 86 when the potentiometer 102 is adjusted with its wiper near the B- output. The resistor 98 is a voltage dropping resistor to lower the dissipation in the transistor 86.

As the wiper on the potentiometer 102 is moved towards the +V output of the power supply 50 the voltage applied to the screen grid of the tubes 36 and 38 is increased and the current flow through the transistor 86 is increased. This causes more current to flow through the resistor 84 and hence causes the transistor 72 to bypass more current around the meter 53.

As a result, the meter 53 will only be subjected to a rise in current therethrough proportional to the power drawn by a patient placed in the field of the coil 30.

The relationship between screen current and plate current of the oscillator tubes is nonlinear. Accordingly, a nonlinear control for the current drawn by the transistor 86 must be provided. To this end, the resistors 88 and 90 are connected together at one end to the wiper of the potentiometer 102. The opposite ends of the resistors are connected together by a diode 92 and the junction of the diode 92 and the resistor 90 are connected by a diode 94 to the output of the zeroing circuit 70.

The diode 92 is off-biased and hence the resistor 90 and diode 94 draw all the current when the wiper of the potentiometer 102 is near the B. Under this condition, transistor 86 receives a first current gain. As the wiper of the potentiometer 102 is moved towards the +V source of potential, a point is reached where the diode 92 is switched into conduction causing the resistor 88 to be connected in parallel with the resistor 90. This then provides a second current gain for the transistor 86. Thus, the nonlinear biasing action of resistors 88 and 90 and diodes 92 and 94 compensate for the nonlinear-ity between screen current and plate current. As a result, the right amount of current is drawn by the transistor 86 causing the transistor 72 to bypass the correct amount of current around the meter 53 to cause the meter 53 to only register power drawn by a patient regardless of the amount of screen voltage applied by the potentiometer 102. The voltage drop between the wiper of the potentiometer 102 and the junction between the resistors 44 nd 46 is very low and can be neglected for all practical purposes.

Thus, it should now be evident that an improved metering circuit for a diathermy is provided wherein a meter monitoring the plate current from the B-lsource of potential indicates the true amount of power being drawn by a patient in the field of the coil 30.

Although one example of the present invention has been shown by way of illustration, it should be understood that there are many other rearrangements and embodiments of the present invention within the scope of the following claims.

I claim:

1. A diathermy the combination comprising:

a vacuum tube tuned plate-tuned grid oscillator circuit, the

vacuum tube thereof having a screen grid and a plate circuit in which high frequency signals are formed;

a coil coupled to the plate circuit for delivering high frequency signals from the plate circuit to a patient undergoing treatment;

a source of plate potential coupled to said plate circuit; and

metering apparatus therefor comprising a meter coupled between said plate circuit for the tube and said source of plate potential, a bypass circuit coupled across said meter for bypassing a preselected amount of current around said meter, and means coupled between said screen grid and said bypass circuit for causing the preselected amount of current drawn by said bypass circuit to change proportional to the current drawn by said screen grid whereby only current above said preselected amount is registered by said meter and thereby cause the meter to register a direct indication of power drawn by a patient undergoing treatment.

2. A diathermy according to claim I wherein said bypass circuit comprises a transistor having the emitter-collector circuit coupled across said meter and means for biasing the base electrode of said transistor.

3. A diathermy according to claim 2 wherein said biasing means comprises a circuit responsive to the current drawn by said screen grid for controlling the bias on the base of said transistor.

4. A diathermy according to claim 2 comprising a source of control potential for said screen grid and wherein the coupling means comprises a further transistor having the emitter-collector circuit thereof coupled to the base electrode of said transistor and the base emitter circuit thereof coupled to said source of control potential for said screen grid.

5. A diathermy according to claim 4 wherein said source of control potential for the screen grid comprises a variable source of potential for said screen grid for controlling the output power delivered by the coil.

6. A diathermy the combination comprising:

a vacuum tube tuned plate-tuned grid oscillator, the vacuum tube thereof having a plate circuit in which high frequency signals are formed and a screen grid;

a coil coupled to the plate circuit for delivering the high frequency signals from the plate circuit to a patient undergoing treatment;

a source of screen potential, the screen grid being coupled to said source of screen potential for control of the power delivered through the coil to the patient;

a source of plate potential coupled to the plate circuit; and

metering apparatus comprising a meter coupled between the plate circuit and said source of plate potential, a bypass circuit having input and output circuits coupled across said meter and a control circuit, and a monitoring circuit having an output circuit coupled to the control circuit of said bypass circuit and a control circuit coupled to said screen grid, the monitoring circuit being responsive to the current flow to said screen grid for controlling the bypass circuit and thereby control the amount of current bypassing said meter through the input/output circuit of said bypass circuit proportional to the current flow to said screen grid.

7. A diathermy according to claim 6 including control means coupled to the control circuit of said bypass circuit and adapted for causing a predetermined minimum current flow through the bypass circuit and thereby allow the meter to be zeroed for minimum screen current flow.

8. A diathermy the combination comprising: a vacuum tube tuned plate-tune gri oscillator, the

vacuum tube thereof having a plate circuit in which highfrequency signals are formed and a screen grid;

a coil coupled to the plate circuit for delivering highfrequency signals from the plate circuit to a patient undergoing treatment;

a source of control potential, said screen grid being coupled to said source of control potential for control of the power delivered through the coil to the patient;

a source of plate potential coupled to the plate circuit; and

metering apparatus comprising a meter coupled between the plate circuit and said source of plate potential, a first transistor circuit having a collector-emitter electrode circuit coupled across said meter, a bias circuit coupled to the base electrode circuit of said first transistor circuit for causing a minimum current flow around said meter for zeroing the same for minimum screen grid current, a second transistor circuit having a first side of the emittercollector electrode circuit thereof coupled to the base electrode circuit of the first transistor circuit, the base electrode circuit of said second transistor circuit being coupled to the source of control potential for the screen grid, a pair of impedances coupled together in parallel and coupled in series between the source of control potential for the screen grid and a second side of the emitter-collector electrode circuit of the second transistor circuit, and a nonlinear conductive device having a predetermined breakdown potential coupled in between one end of each of the pair of impedances to thereby provide a variable bias for control of the base circuit of the second transistor circuit thereby causing the second transistor circuit to vary the current bypassed by the first transistor circuit causing current to flow through the meter proportional to power drawn only by the patient undergoing treatment.

9. A diathermy the combination comprising:

an oscillator circuit;

a coil, the oscillator circuit having an output circuit coupled to said coil for delivering high-frequency signals through said coil to a patient undergoing treatment and including a control input at which control signals are applied which control the magnitude of the signals delivered through said coil to a patient;

a source of power coupled to said oscillator circuit for providing the power delivered to said coil; and

metering apparatus comprising a meter coupled between said oscillator output circuit and said source of power, a controllable bypass circuit coupled across said meter, and having a control input, means coupled between said oscillator control input and said bypass circuit control input and being operative for causing said bypass circuit to bypass a reselected amount of current around said meter which is proportional to the control signals at said oscillator control input thereby leaving only current above said preselected amount being registered by said meter, said meter thereby registering a direct indication of power drawn by a patient undergoing treatment.

10. A diathermy according to claim 9 including a low end scale expanding circuit for said meter comprising a first impedance coupled in series with said meter, and a second impedance and nonlinear conductive device connected together in series and coupled in parallel with said meter and first impedance circuit, the nonlinear conductive device having a threshold potential such that current through such parallel connection passes exclusively through the first impedance and such meter until the current through said first impedance causes the threshold potential of said nonlinear conductive device to be exceeded with subsequent current flow therethrough.

l t l l '8 

1. A diathermy the combination comprising: a vacuum tube tuned plate-tuned grid oscillator circuit, the vacuum tube thereof having a screen grid and a plate circuit in which high frequency signals are formed; a coil coupled to the plate circuit for delivering high frequency signals from the plate circuit to a patient undergoing treatment; a source of plate potential coupled to said plate circuit; and metering apparatus therefor comprising a meter coupled between said plate circuit for the tube and said source of plate potential, a bypass circuit coupled across said meter for bypassing a preselected amount of current around said meter, and means coupled between said screen grid and said bypass circuit for causing the preselected amount of current drawn by said bypass circuit to change proportional to the current drawn by said screen grid whereby only current above said preselected amount is registered by said meter and thereby cause the meter to register a direct indication of power drawn by a patient undergoing treatment.
 2. A diathermy according to claim 1 wherein said bypass circuit comprises a transistor having the emitter-collector circuit coupled across said meter and means for biasing the base electrode of said transistor.
 3. A diathermy according to claim 2 wherein said biasing means comprises a circuit responsive to the current drawn by said screen grid for controlling the bias on the base of said transistor.
 4. A diathermy according to claim 2 comprising a source of control potential for said screen grid and wherein the coupling means comprises a further transistor having the emitter-collector circuit thereof coupled to the base electrode of said transistor and the base emitter circuit thereof coupled to said source of control potential for said screen grid.
 5. A diathermy according to claim 4 wherein said source of control potential for the screen grid comprises a variable source of potential for said screen Grid for controlling the output power delivered by the coil.
 6. A diathermy the combination comprising: a vacuum tube tuned plate-tuned grid oscillator, the vacuum tube thereof having a plate circuit in which high frequency signals are formed and a screen grid; a coil coupled to the plate circuit for delivering the high frequency signals from the plate circuit to a patient undergoing treatment; a source of screen potential, the screen grid being coupled to said source of screen potential for control of the power delivered through the coil to the patient; a source of plate potential coupled to the plate circuit; and metering apparatus comprising a meter coupled between the plate circuit and said source of plate potential, a bypass circuit having input and output circuits coupled across said meter and a control circuit, and a monitoring circuit having an output circuit coupled to the control circuit of said bypass circuit and a control circuit coupled to said screen grid, the monitoring circuit being responsive to the current flow to said screen grid for controlling the bypass circuit and thereby control the amount of current bypassing said meter through the input/output circuit of said bypass circuit proportional to the current flow to said screen grid.
 7. A diathermy according to claim 6 including control means coupled to the control circuit of said bypass circuit and adapted for causing a predetermined minimum current flow through the bypass circuit and thereby allow the meter to be zeroed for minimum screen current flow.
 8. A diathermy the combination comprising: a vacuum tube tuned plate-tuned grid oscillator, the vacuum tube thereof having a plate circuit in which high-frequency signals are formed and a screen grid; a coil coupled to the plate circuit for delivering high-frequency signals from the plate circuit to a patient undergoing treatment; a source of control potential, said screen grid being coupled to said source of control potential for control of the power delivered through the coil to the patient; a source of plate potential coupled to the plate circuit; and metering apparatus comprising a meter coupled between the plate circuit and said source of plate potential, a first transistor circuit having a collector-emitter electrode circuit coupled across said meter, a bias circuit coupled to the base electrode circuit of said first transistor circuit for causing a minimum current flow around said meter for zeroing the same for minimum screen grid current, a second transistor circuit having a first side of the emitter-collector electrode circuit thereof coupled to the base electrode circuit of the first transistor circuit, the base electrode circuit of said second transistor circuit being coupled to the source of control potential for the screen grid, a pair of impedances coupled together in parallel and coupled in series between the source of control potential for the screen grid and a second side of the emitter-collector electrode circuit of the second transistor circuit, and a nonlinear conductive device having a predetermined breakdown potential coupled in between one end of each of the pair of impedances to thereby provide a variable bias for control of the base circuit of the second transistor circuit thereby causing the second transistor circuit to vary the current bypassed by the first transistor circuit causing current to flow through the meter proportional to power drawn only by the patient undergoing treatment.
 9. A diathermy the combination comprising: an oscillator circuit; a coil, the oscillator circuit having an output circuit coupled to said coil for delivering high-frequency signals through said coil to a patient undergoing treatment and including a control input at which control signals are applied which control the magnitude of the signals delivered through said coil to a patient; a source of power coupled to said oscillator circuit for providing the power delivereD to said coil; and metering apparatus comprising a meter coupled between said oscillator output circuit and said source of power, a controllable bypass circuit coupled across said meter, and having a control input, means coupled between said oscillator control input and said bypass circuit control input and being operative for causing said bypass circuit to bypass a reselected amount of current around said meter which is proportional to the control signals at said oscillator control input thereby leaving only current above said preselected amount being registered by said meter, said meter thereby registering a direct indication of power drawn by a patient undergoing treatment.
 10. A diathermy according to claim 9 including a low end scale expanding circuit for said meter comprising a first impedance coupled in series with said meter, and a second impedance and nonlinear conductive device connected together in series and coupled in parallel with said meter and first impedance circuit, the nonlinear conductive device having a threshold potential such that current through such parallel connection passes exclusively through the first impedance and such meter until the current through said first impedance causes the threshold potential of said nonlinear conductive device to be exceeded with subsequent current flow therethrough. 